The goals of this project are twofold: 1) the characterization of Clq receptor (ClqR) expression on human polymorphonuclear (PMN) and mononuclear phagocytes, and 2) investigation into the modulation of Clq of FcR- and complement receptor-mediated phagocytosis by monocytes and macrophages. Elutriated monocytes were cultured for seven days in Teflon flasks ("macrophages") containing HL1 (a serum-free medium) or RPMI with 12% fetal bovine serum (FBS). Macrophages were adhered to human serum albumin (HSA)- or Clq-coated LabTek chamber slides, and the ingestion of sheep erythrocytes opsonized with IgM and C4b(EAC4b) was measured. EAC4b are bound to and are ingested via CR1. HL1-cultured macrophages did not ingest EAC4b whether the macrophages were adherent to HSA or Clq. RPMI/12% FBS-cultured, HSA-adherent macrophages also did not ingest EAC4b; however, Clq adherent macrophages did phagocytose EAC4b. This suggested that Clq, in the presence of other factors, could activate CR1 for ingestion. Because tumor promoting phorbol esters, such as phorbol dibutyrate (PDBu), activate CR1-mediated phagocytosis by macrophages, we investigated the effects of Clq on PDBu-stimulated phagocytosis in HL1-cultured macrophages. In the presence of PDBu, Clq- adherent macrophages markedly increased the ingestion of EAC4b over that level measured for macrophages treated with PDBu alone. The concentration of PDBu and time of stimulation were important determinants of maximal synergy with Clq. This Clq- stimulation of monocytes was likely mediated via ClqR, because the effect was reproduced by purified Clq tail fragments and blocked by F(ab')2 anti-Clq. Furthermore, freshly isolated monocytes, which have not been reported to ingest via CR1 even when stimulated with PDBu, became competent to phagocytose EAC4b when stimulated concurrently by PDBu and Clq.